The Gowanus Canal is a 1.8-mile-long waterway connecting Upper
New York Bay (the bay in between Brooklyn, Manhattan, New Jersey,
and Staten Island) with the formerly industrial interior of
Brooklyn. Originally it was fed by the marshland and freshwater
springs in Brooklyn and drained into the Atlantic Ocean in Upper
New York Bay;

Because of the way it was created, though, it has become stagnant
and polluted by decades of runoff and dumping from local
neighborhoods and businesses. In the summer, you can smell it
from blocks away. It's not a good smell. The post notes some
pretty horrible things about the canal:

The Gowanus is literally a sewer. First, small businesses
along the canal are illegally dumping sewage and wastewater into
it. There are also raw sewage tanks at the end of the canal,
maintained by the city, that overflow right into the Gowanus.

It should kill most marine life in it because the water
doesn't contain enough oxygen. Somehow even under these toxic
conditions, bacterial life survives.

The bubbles that float up from the lake come from decomposing
sewage at the bottom.

It's a superfund site and the EPA has a plan to clean it up.
The cleanup is expected to cost $500 million, but will probably
end up costing much more.

Giant white clumps of bacteria — called biofilms — float near
the bottom of the canal.

We don’t really have any idea of the things that are growing
in it. Even the EPA is clueless. There could be entirely new
species growing in there.

Things that we know are in there? Heavy metals including
arsenic at 60 times the healthy exposure levels, carcinogens
(that cause cancer), and congeners (a type of toxic chemical that
come from factory runoff).

The EPA also has no idea about the long-term effects on
people who live near the canal, and no one is studying it. There
is anecdotal examples that residents succumb early to cancer,
Nosowitz writes. That's just from breathing in fumes.